OBJECTIVE Genetically engineered bacterial strains were developed and used as live, oral vaccines to immunize against mucosal SIV infection in rhesus macaques. RESULTS We developed recombinant Salmonella typhimurium strains that express the SIV p27 antigen from a chromosomally-inserted expression construct. Live Salmonella were delivered by intragastric intubation of juvenile rhesus macaques and immune responses to the bacterial vector and the recombinant protein were characterized. In parallel animals received intramuscular immunization with purified, recombinant p27 antigen in polyphosphazene adjuvant. Intragastric immunization with recombinant bacteria elicited strong lymphoproliferative responses to Salmonella and to the viral protein without generating a measurable serum immunoglobulin response. Intramuscular immunization with soluble viral protein failed to elicit lymphoproliferative responses but generated high levels of serum antibody. Intragastric immunization did not boost the serum immunoglobulin response initiated by prior intramuscular immunization. Likewise, intramuscular boosting did not increase levels of lymphoproliferative responses to viral antigen. Immunized animals were challenged with a defined SHIV89.6PD inoculum and outcomes of challenge were assessed for information regarding viral immunity mechanisms. FUTURE DIRECTIONS Test the capacity for Salmonella strains to deliver DNA constructs to mucosal tissues. Develop additional bacterial strains expressing other viral antigens. Test combination vaccination strategies. KEY WORDS AIDS, vaccine, mucosal, recombinant bacteria, Salmonella typhimurium FUNDING NIH R01 AI36643 and RR00167 PUBLICATIONS Pauza, C.D., D. Horejsh, and M. Wallace. 1998. Mucosal transmission of virulent and avirulent lentiviruses in macaques. AIDS Res. Human Retrovir. 14:S83-87. [J] Steger, K.K., M. Dykhuizen, J. Mitchen, P. W. Hinds, B. L. Preuninger, M. Wallace, J. Thomson, Y. Lu, and C.D. Pauza. 1998. CD4+ T cell and CD20+ B cell changes that Predict Rapid Disease Progression after SHIV89.6PD Inoculation of Rhesus Macaques. J. Virol. 72:1600-1605. [J]